OBJECTIVE: To study and characterize in vivo the morphology and physiology of the vascularization and invasiveness of malignant tumors transplanted into the hamster cheek pouch. APPROACH: Malignant tumors are transplanted into a hamster cheek pouch which has been fitted with the transparent chamber described by Sanders and Shubik. The growth of the neo-vessels arising from the host is observed daily for various periods of time. The vascular pattern and the individual cell morphology of these vessels invading the transplanted tumor are observed by light and electronmicroscopy (TEM & SEM). The relationships between the growing tumor and its vascular supply are studied.